Hi! That’s the pass which marks VSM pages for update. In some cases where there is geometry close below the water the pages will already be updated so there’s no difference, but it’s not something that can be relied on. If VSM page marking isn’t done properly it can result in extremely blocky shadows. Also we have plans to use the depth prepass for other improvements, like the recently added depth occlusion mask https://github.com/EpicGames/UnrealEngine/commit/672ebfc3e3065bf6bc2cabbb593ab64a3094cade
// Kevin